The present invention relates generally to the gaseous diffusion process utilized for the separation of uranium isotopes, and more particularly for a method continually monitoring the stack gases emanating from purge cascades of a gaseous diffusion plant for possible uranium activity.
The enrichment of uranium in the 235 isotope has been achieved for many years in gaseous diffusion systems wherein uranium in a gaseous state, primarily in the form of uranium hexafluoride (UF.sub.6), is passed through a series of diffusion barriers to selectively separate the 235 isotope from the 238 isotope until desired enrichment of the uranium in the 235 isotope is achieved. In the gaseous diffusion process one or more purge cascades are used in conjunction with the main enrichment cascade for the purpose of removing nitrogen and other diluents from the uranium hexafluoride stream. These purge cascades differ from the main process cascade due to the separation of the diluents from the uranium hexafluoride and the relative concentration of the isotopic forms of the uranium hexafluoride which remain substantially constant while passing through the purge cascades. In the main cascade the density of the gas remain fairly constant from point to point but in the purge cascade the density varies from that of a mixture having a small amount of nitrogen or other diluent gas in a large amount of uranium hexafluoride at the feed point to a stream of substantially pure diluent gas at the product end. These gases remaining at the product end are routinely discharged into the atmosphere through a stack.
While it is the intent of the purge cascade to remove all of the radioactive material from the gases before they are discharged into the atmosphere some uranium hexafluoride may periodically remain in the stack gases so as to present a possible environmental contamination problem. Previously, the monitoring of the stack gases for radioactive activity was achieved by periodically taking samples of the stack gases at selected locations in the exit lines of the purge cascade. These samples were then transported to an analytical laboratory for determining the uranium content of the stack gases. This somewhat cumbersome procedure presented some problems due to the time lag from taking the sample to determining the presence of radioactive activity in the gases and also presented a considerable potential hazard in that large quantities of radioactive material could escape into the atmosphere between samplings.